What is your expert opinion on bullies?

Low self esteem. It is always someone who feels insecure inside who pick on someone else.
Bully once bullied. Bullies usually have a traumatic past, are temperamentally defiant as children and develop a sense of low esteem which they project out try to make others feel lowly. Most of the time they have been bullied them selves. They usually suffer from mood disorder significantly dysphoria.
Inferiority. Believe it or not, bullies tend to have inferiority complexes, often from having been bullied themselves. and yes, it can continue into adult life. Tell someone you know and trust about the bullying. Keeping it inside does no good.